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    <title>Document</title>

  <title> 
     Document
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  <style>
    #top {
      background-color:aqua;
      color:tomato;
      height: 200px;
      /*垂直居中*/
      line-height: 200px;
        /*水平居中*/
      text-align: center;
    }


    <!--
    .top{
      background-color: aqua;
      color: blueviolet;
      height: 200px;
      /*弹性布局*/
      display: flex;
      /*垂直居中*/
      align-items: center;
      /*水平居中*/
      justify-content: center;
    }
  -->


  </style>
</head>
<body>
    <h1 id="top">
      选题系统
    </h1>
    <div id="middle">
      <a href="about.html">关于我们</a>
    </div>
</body>
</html>